Move documentation generation into WeaselDiesel

It's currently in https://github.com/mattetti/wd-sinatra, but it makes more sense in WD itself. This isn't ready to be merged, it's mostly a quick and dirty proof of concept. We should probably discuss whether or not this is the right approach.

In wd_sinatra, it was defined as a rake task. Here I changed it into a weasel_diesel CLI via Thor. It takes 1 required argument, which is the source path, and a second optional argument, which is doc destination path. I had to stub in an implementation method in WeaselDiesel, so that the doc generator could parse the web service definitions.

If you wanted it to read your ruby api files in the `api` directory, and put the doc in the `doc` directory, you would use it like:

```bash
weasel_diesel api doc
```

I ran it against my local test wd_sinatra app, and it seems to work.